1. Field of the Invention
The present invention relates to a contactless integrated circuit (hereinafter IC) card, and more particularly, to a contactless IC card containing a built-in coil antenna for transmitting and/or receiving signals to and from an external device.
2. Description of Related Art
Conventionally, IC cards are widely used to control access to a variety of facilities, to record consumer transactions, and to function as a form or medium of exchange, among other applications
Such cards, which perform the dual functions of transceiving signals and also of receiving power from the external device, are of two types: A contact IC card, requiring physical contact between the IC card and the external device in order to power the IC card and to exchange information with the external device, and a contactless IC card, which is powered by a process of electromagnetic induction from the external device and which exchanges information with the external device by transmitting and receiving electromagnetic waves (radio waves) to and from the external device.
Such contactless IC cards operate by using a wire-wound coil antenna for gaining power from the external device and for transmitting and/or receiving electromagnetic signals to and from the external device.
In an earlier phase of the conventional art, the wire-wound coil antenna was formed by printing or electroplating a conductive film.
However, there are several disadvantages to this process, including the fact that the electrical resistance of such a film is generally greater than at least that of a copper wire coil, which is undesirable.
As a result, wound coils of copper wire came to replace the electroplated films.
In order to further facilitate an understanding of the present invention, a description will first be given of a conventional contactless IC card using such a wound coil of copper wire as an antenna.
FIG. 1 is an exploded view of a conventional contactless IC card. FIG. 2 is a top view of an inlet laminate (hereinafter inlet laminate) of a conventional contactless IC card. FIG. 3 is a diagram illustrating the problem with the conventional contactless IC card.
As can be seen from the diagrams, a contactless IC card 1 comprises an inlet laminate 2, plastic laminates 3 and 4 (hereinafter plastic laminates), an IC module 5, and a wire-wound coil antenna 6 (hereinafter wire-wound coil 6).
The IC module 5 and the wire-wound coil 6 are provided on the inlet laminate 2. The IC module 5 is fixedly mounted on the inlet laminate 2 using an adhesive agent. Additionally, the IC module 5 has a pair of electrodes 7, 8 to which the wire-wound coil 6 is connected.
Additionally, the wire-wound coil 6 is formed into a loop around the outer edge of the inlet laminate 2. The wire-wound coil 6 is designed to have an inductance and number of turns so as to resonate at a predetermined frequency.
As shown in FIG. 3, end portions 6A, 6B of the wire-wound coil 6 are attached by soldering 9 to the electrodes 7, 8 provided on the IC module 5. The end portions 6A, 6B are bent at substantially a right angle to the coil portion of the wire-wound coil 6 and connected to the electrodes 7, 8 of the IC module 5, with the end portions 6A, 6B forming essentially a straight line in the vicinity of the IC module 5.
The inlet laminate 2 having the structure described above is enclosed by a pair of plastic laminates 3, 4 to which the inlet laminate 2 is thermobonded, such that the three laminates together form a single unit with the IC module 5 and the wire-wound coil 6 imbedded within the structure so formed by the three laminates.
The IC card 1 may be of a type that is used either by attachment to an external device not shown in the diagrams (for example a reader/writer) or used in proximity to the external device, as indicated above. In either use state, electrical power is supplied to the IC card 1 by a process of electromagnetic induction from the external device to the IC card 1, and further, information is transmitted between the IC card 1 and the external device by use of electromagnetic waves (i.e., radio waves).
However, a disadvantage of the conventional IC card 1 having the structure described above is that, typically, the IC card 1 is carried from place to place and used in a variety of conditions in which there is a great deal of variation in temperature. For example, an IC card left inside a car exposed to the midsummer sun can be exposed to temperatures exceeding 50xc2x0 C. Conversely, depending on the climate, in winter the IC card could be exposed to temperatures as low as several tens of degrees below 0xc2x0.
Accordingly, such IC cards are exposed to temperature cycle tests in order to test their reliability, the temperature cycles ranging from a low of xe2x88x9230xc2x0 C. to room temperature, and from room temperature to 70xc2x0C.
However, when subjected to such stress tests many of the conventional IC cards such as the IC card 1 described above exhibit the following problem, in that the straight part of the end portions 6A, 6B of the wire-wound coil 6 tends to snap at a point indicated substantially as B in FIG. 3.
The above-describing parting or breakage in the wire-wound coil appears to be due to several factors, as follows:
1. There is a sharp difference in the coefficient of thermal expansion (hereinafter referred to as a coefficient of thermal expansion mismatch) between the copper wire that forms the wire-wound coil 6 and the laminates 2-4. The coefficient of thermal expansion of the copper wire is 17xc3x9710xe2x88x926 and the coefficient of thermal expansion of the plastic substrate is 50xc3x9710xe2x88x926.
2. The IC module 5 to which the ends of the coil 6A, 6B are connected is imbedded within the laminates 2-4 which are thermobonded together, and hence the IC module 5 cannot move.
3. The portions of the end portions of the coil 6A, 6B near the IC module 5 are straight, so the coil end portions 6A, 6B cannot absorb the tension generated at the coil end portions 6A, 6B due to the above-described coefficient of thermal expansion mismatch.
Additionally, in the process of manufacturing the above-described IC card 1 the laminates 2-4 are thermobonded together, a process which also imparts tension to the wire-wound coil 6. Therefore breakage of the end portions 6A, 6B of the wire-wound coil occurs during this thermobonding process as well.
Accordingly, it is an object of the present invention to provide an improved and useful contactless IC card in which the above-described disadvantage is eliminated, and more specifically, to provide an improved and useful contactless IC card in which changes in ambient temperature do not cause the wire-wound coil to snap.
The above-described object of the present invention is achieved by an improved contactless IC card of the type having one or more laminate members, a wire-wound coil provided on the laminate member for receiving power and for transmitting and receiving signals to and from an external device, and an IC module for inputting and outputting the signals, the wire-wound coil being coupled to the IC module, wherein the improvement comprises an end portion of the wire-wound coil being formed into a buffer portion near the point at which the wire-wound coil is coupled to the IC module so as to absorb tension generated in the wire-wound coil.
According to this aspect of the invention, the buffer portion absorbs tension generated in the wire-wound coil due to the coefficient of thermal expansion mismatch between the copper wire that forms the coil and the surrounding plastic laminates.